Hi there!
Check to see that the movie extension is accepted with both Dropbox and Cinex? See the following below, this is directly from the Dropbox site:
Native file types
There are a number of file formats that can be opened natively on the iPad by simply selecting the file from the file browser. You can view these in the Dropbox app by simply tapping them from the primary Dropbox tab.
Images (.jpg, .tiff, .gif)
Music (.mp3, .aiff, .m4a, .wav)
Movies (.mov, .mp4, .m4v)
Microsoft Word documents (.doc, .docx)
Microsoft PowerPoint presentations (.ppt, .pptx)
Microsoft Excel spreadsheets (.xls, .xlsx)
Adobe PDF (.pdf)
Keynote presentations (.key)
Pages documents (.pages)
Numbers spreadsheets (.numbers)
Web pages (.htm, .html)
Text and rich-text files (.txt, .rtf, and many more)